1. Field of the Invention
The present invention relates to plasma assisted deposition of thin, homogeneous films from gaseous precursors onto temperature stable substrates with large surface areas and in particular to a new method and device for Plasma assisted Chemical Vapor Deposition (PCVD) of diamond films, diamond-like films and similar materials on large area quartz, glass and ceramic substrates.
2. Information Disclosure Statement
The unique characteristics of diamond and diamond-like films (DF and DLF) have begun to be recognized and used in the development of flat panel displays, of heat sinks for powerful lasers, of cold cathodes having high current densities, etc. Characteristics of the films can be modified through the use of mixtures of working gases to provide carbon moieties and other elements for `molecular alloys.` For example, methane, acetylene, carbon monoxide, carbon dioxide, alcohol vapors, carbon tetrachloride and tetrafluoride can serve as carbon sources, typically in an argon carrier gas containing oxygen or hydrogen to produce the diamond or diamond-like films. There remain many problems with achieving preselected properties for diamond and diamond-like films deposited on substrates with large surfaces, e.g. &gt;100 cm.sup.2, especially specific thickness patterns, including uniform thickness over large to very large surfaces, &gt;1000 cm.sup.2.
The current problems with the prior art are well summarized in Pinneo (U.S. Pat. No. 5,449,412). The commercial utility of microwave enhanced CVD deposition of diamond films is limited due to difficulty in coverage of large areas due to small plasma size and presence of large thermal gradients which degrade deposition uniformity. These problems adversely affect the economics and limit the range of applications of PCVD systems for many materials beyond diamond depositions.
Small plasma size creates serious problems because the deposition process varies strongly with respect to proximity to the plasma for numerous critical parameters, such as growth rate, diamond quality and surface structure. The nonuniformities which arise strongly affect the economic and technical feasibility of manufacturing products having diamond films by microwave plasma CVD process.
One attempt to treat larger areas by PCVD techniques is by Bou et al. (U.S. Pat. No. 5,360,485). Here the deposition is remoted from the space where initial plasma formation occurs. The deposition zone extends outside the waveguide to provide a larger area for deposition to occur and to reduce thermal exposure of substrate. Substrates still are heated to about 1000.degree. C. The configuration is essentially a cone shape. Relatively flat and long substrates cannot be coated with this set up because of the difficult geometry, however, as described within the patent, large numbers of small substrates could be handled simultaneously, i.e. for preparing small sample substrates, large total areas are possible to coat uniformly. In summary then, larger areas can be coated, but only for small individual pieces or for special rounded shapes like cones. It is not clear how controlled or uniform the deposition can be since the properties of the plasma may change along the path as described in the patent.
Most of the other prior art which is trying to expand the area that can be coated use a ball plasma as their starting point. The general limitation of such plasmas is that as the plasma energy is increased, the size of the plasma ball diameter increases, moving the center of the plasma further away from the substrate. For example, Smith in EPO 0480581A forms a plasma ball over a substrate to be covered. It is clear from geometric considerations alone only a small part of surface of the plasma can be in close proximity to the substrate surface to be coated. Since the majority of the plasma volume becomes more distant from the substrate surface as the power of the plasma is increased for high rate deposition conditions, Smith introduces high speed recirculating of the reactive gas mixture around the support holding the substrate in contact wt. the plasma. The main problem, being attacked by Smith, is the lifetime of the reactive species becoming shorter than the path to the substrate. The supersonic flows, described in this patent and figures, must be turbulent in the vicinity of the surface to be coated, which gives rise to increasing difficulty in achieving uniform deposition conditions across any section of the substrate surface which is being coated.
Sevillano et al. in U.S. Pat. No. 5,405,645 deals with analyzing the composition of a ball plasma by spectrally following the C.sub.2 formed in relation to the concentration of hydrogen atoms in the plasma. Using 5 kW of power, they found ratios of C.sub.2 /H which corresponded to high rates of deposition of diamond-like coatings on their substrates. Its main emphasis is simply on achieving deposition rates of &gt;10 mg/hr. It is noted that the deposition is enhanced typically at the center of the substrate, which is closest to the plasma. Powerful plasmas can deposit diamond or diamond-like coatings over broad areas and at high rates but the thickness will vary generally inversely with the square of distance from the center of the substrate being coated. High rates are achieved but one is left with a strongly varying thickness across the coated area.
A good attempt to solve the problems was suggested in U.S. Pat. No. 5,449,412 where a ball plasma is scanned across a substrate's surface to yield a deposition area significantly larger than achieved by other methods. There is some discussion within this reference of mixing a microwave and a reflection of the microwave to position the active plasma region within a reaction tube. The solution offered in this patent still has potential problems of non-uniformity in the deposited coating across the initial ball-like deposition area as well as along the scanned direction. Also the scan parameters and those of the plasma must remain constant over the time of the deposition or be able to be varied very consistently if a uniform coated area is to be produced. The modulation of the microwaves as they approach and exit the horns merely attempts improvement in one dimension, a ball plasma is still the base configuration for the deposition. Neither coating of really large (&gt;100 cm.sup.2) substrate areas nor highly uniform, preselected coating thicknesses are currently able to be achieved reliably and economically.
In summary, the prior art has continued to approach several of the problems associated with achieving highly controlled depositions over large surfaces but without complete success, especially as to uniform properties over even large sections of very large surfaces. The present invention is designed to provide solutions to these problems.